ride height question

The method of setting ride height spelled out by the FSM completely removes tire height out of the equation, it was specifically designed to.

The FSM essentially sets the angle of the lower control arms. If you change the tire height the car will be higher or lower from the ground, but the suspension remains at the same angles and the distances from the control arms to the bump stops remain the same. The suspension geometry is what the FSM spec’d.

Now, is the FSM spec still relevant for you? Depends on two things primarily- whether or not you use radial tires and what size torsion bars you run. And then of course you should also consider how you’re using the car, as always.

If you run radials, the FSM spec is no longer ideal. The control arm angles at ride height set how your caster/camber curves start out, and those curves are better suited for bias plys using the stock configuration. If you run radials you want negative camber gain for best handling, and that’s not what you get with the factory ride height specs.

You can get negative camber gain if you set the ride height so that the angle of the control arms is almost parallel to the ground. That represents and significant drop in ride height though, basically it makes A-B zero. And in order to pull that off you also need larger torsion bars. And depending on how big your torsion bars are, you also need to shorten the lower bump stop and raise the upper bump stop to re-center the range of travel.

Now, if maximum handling isn’t your primary goal, then you don’t need to do all that. But you also don’t need to use the FSM spec, as long as you leave yourself enough suspension travel for the size of the torsion bars you’re running. If you’re running stock torsion bars you better stick close to the FSM spec or you’ll bottom the suspension out all the time. If you run larger diameter bars than stock, then you can lower the car some and improve your suspension geometry and handling.

One thing I would NOT do, regardless the use of the car, is raise it up higher than the FSM spec. The suspension geometry goes to hell if you do that, so unless you’re trying to make your car a 4x4 I’d set it at the FSM spec or lower. If all you’re worried about is straight line stability then the rake only matters as much as it effects your caster. If you can get good positive caster it doesn’t matter if there’s rake for straight line. Now of course for turning stability you’d want your roll centers at close to the same height.